Page 11 - Visions of the Future Chemistry and Life Science

2 G. ROBERTS

1.2 The interaction of intense femtosecond laser light with
molecules

The interaction of femtosecond laser light with atoms and molecules is
completely different to that involving longer laser pulses. This arises from
the ultrashort duration of femtosecond laser pulses, which is faster than
the characteristic dynamical time scales of atomic motion, and their ultra-
high intensity, which initiates a whole range of unprecedented phenom-
ena. What exactly happens when an intense, ultrafast laser beam irradiates
a sample of molecules depends crucially on the intensity of the laser,
which determines the number of photons supplied to an individual mole-
cule and can contort the allowed energy levels of the molecule; also impor-
tant is the frequency of the laser, which, together with the intensity,
affords optical access to different molecular energy states. The detailed
physics of the light–matter interaction will of course also depend on the
structure of the irradiated molecule, but whatever its identity, certain
general features of the excitation of atoms and molecules by ultrafast laser
photons have emerged from pioneering studies by research groups through-
out the world.
First to respond to the laser field are the lighter electrons, which do so
on a time scale of attoseconds (a thousandth of a femtosecond): depending
upon the intensity of the incident light, the one or more photons absorbed
by the molecule either promote an electron to a high-lying energy state of
the molecule, or the electron is removed from the molecule altogether,
leaving a positively charged ion; at very high intensities multiple electron
excitation and ionisation through various mechanisms can occur. Over a
far longer time scale of tens or hundreds of femtoseconds, the positions of
the atomic nuclei within the molecule rearrange to accommodate the new
electrostatic interactions suddenly generated as a result of the new elec-
tronic state occupancy prepared by the ultrafast laser pulse: the nuclear
motions may involve vibrations and rotations of the molecule, or the mole-
cule may fall apart if the nacent forces acting on the atoms are too great to
maintain the initial structural configuration. In addition, at high incident
intensities, the electric field associated with the laser beam distorts the
electrostatic forces that bind the electrons and nuclei in a molecule to such
an extent that the characteristic energy levels of the molecule are modified
during the ultrashort duration of the laser pulse.
Each of the above phenomena is the subject of intensive research pro-